Conventionally, various techniques as follows are widely used. A raw material gas obtained by adding a nitrogen gas in an amount of several thousand ppm or more to an oxygen gas is supplied to an ozone generator to generate a highly concentrated ozone gas, and in the field of semiconductor manufacture, the resulting highly concentrated ozone gas is commonly used in an ozone treatment step such as formation of ozone-oxidized insulating films or ozone cleaning. In the field of semiconductor manufacture or the like, when an ozone gas is supplied to a multi-ozone treatment apparatus including multiple ozone treatment apparatuses, it is generally conceivable to construct an ozone gas supply system (unit) in which a plurality of ozone generating mechanisms (means), each including, for example, an ozone generator, an ozone power source, and a flow rate controller (MFC), are provided in correspondence with a plurality of ozone treatment apparatuses, and the ozone generating mechanisms independently supply an ozone gas to the corresponding ozone treatment apparatuses.
As shown in FIG. 9, conventionally, in order to increase the efficiency of generating an ozone gas by an ozone generator 71 that includes electrodes 71a and 71b and a dielectric 71c and receives power supplied from an ozone power source 72, a typical oxygen gas contains a nitrogen gas of approximately 50 to several thousand ppm. In the case of a high-purity oxygen gas with low nitrogen content (less than 50 ppm), a trace amount (500 ppm or more) of N2 gas is added to the ozone generator together with the high-purity oxygen gas.
If the raw material oxygen gas contains a N2 gas in an amount of 500 ppm or more, highly concentrated ozone is generated by a catalytic reaction of a trace amount of NO2 generated by a discharge reaction shown in FIG. 10. In particular, addition of 500 to 20000 ppm of nitrogen gas increases the efficiency of generating ozone by a catalytic reaction of a trace amount of nitrogen dioxide generated by discharge, resulting in generation of mostly highly concentrated ozone. It has been verified by experiments that a raw material gas with an amount of nitrogen added of 500 to 20000 ppm is an optimal condition in the performance of generating ozone.
As shown in (1) to (3) below, the discharge reaction shown in FIG. 10 allows generation of highly concentrated ozone, using raw material oxygen O2, photoelectric discharge light, and a trace amount of NO2 catalytic gas.
(1) Generation reaction of a trace amount of NO2 gas by discharging                Ionization reaction of nitrogen moleculesN2+e2N+        Generation reaction of NO2 2N++O2+MNO2         
(Generation of several ppm to several ten ppm of NO2 gas)
(2) Generation of oxygen atoms O by a catalytic effect of NO2 by discharge light                Photodissociation reaction of NO2 NO2+hvNO+O        Oxidation reaction of NONO+O2 (raw material oxygen)NO2+O        Through the above two reactions, NO2 acts as a catalyst to generate oxygen atoms        
Ozone O3 is generated by reaction of a large number of oxygen atoms O generated by the reaction (2) with oxygen gas molecules O2.
(3) Generation of ozone O3 (three-body collision)R2;O+O2+M→O3+M
The reactions (1) to (3) produce highly concentrated ozone.
However, because the raw material oxygen gas contains a large amount of N2 gas, nitric acid and an NOx by-product gas such as N2O5 and N2O are also generated, in addition to the ozone gas, by silent discharge in the ozone generator. Specific chemical formulas for the generation of a NOX by-product gas and nitric acid are as follows.N2+eN2*+eN2+hv (310, 316, 337, 358 nm)
N2*; excitation of nitrogen
Ultraviolet light caused by a nitrogen gasH2O+eH+OH+e (electrolytic dissociation of water vapor)N2+e2N−+e (electrolytic dissociation of nitrogen molecules)NO2+hv (295 to 400 nm)NO+O(3P)H+O2+MHO2+MHO2+NOOH+NO2 N2O5+H2O2HNO3 OH+NO2+MHNO3+M
As described above, the NOX by-product gas and nitric acid are also generated in addition to the ozone gas.
If a large amount of NOX by-product is generated, nitric acid (HNO3) clusters (water vapor) are generated by reaction of an NOX gas component and moisture contained in the raw material gas, and an ozonized gas is obtained in which oxygen, an ozone gas, and a trace amount of NOX gas and nitric acid clusters are mixed. If the trace amount of nitric acid clusters contained is several hundred ppm or more, rust of chromium oxide or the like is deposited on the inner surface of a stainless steel pipe serving as an ozone gas outlet pipe by the nitric acid, and metal impurities are introduced into a clean ozone gas. If such an ozonized gas is used as a reaction gas for semiconductor manufacturing apparatuses, the metal impurities will adversely affect the manufacturing of semiconductor apparatuses, and the trace amount of generated nitric acid clusters will also adversely affect, as a reaction poison, treatments such as “etching treatment of silicon oxide films with ozone,” and “cleaning wafers or the like with ozone water” performed by the semiconductor manufacturing apparatuses.
Also, an ozone gas supply system including an ozone generator, an ozone power source, and the like is generally provided with an ozone generator, an ozone power source, a raw material gas piping system that supplies an ozone gas or a raw material gas to the ozone generator via flow rate adjusting means such as a MFC that controls the flow rate of the ozone gas or the raw material gas, pressure adjusting means such as an APC that controls the atmospheric pressure of a gas contained in the ozone generator, an ozone concentration detector that detects the concentration of the ozone gas output from the ozone generator, and output gas piping systems including ozone flow meters, the number of the output gas piping systems corresponding to the number of multiple ozone treatment apparatuses.
However, it is not possible to supply a large amount of highly concentrated ozonized oxygen having a very small amount of NOX by-product. Moreover, a very large space is required to construct such an ozone generating system provided with multiple ozone treatment apparatuses as described above. In the case of constructing an ozone-gas supply system by performing overall control on the multiple ozone treatment apparatuses, the system becomes large, causing many problems in terms of cost, installation space, and the like, as well as disadvantages in practical use.
Accordingly, an attempt was made to generate ozone using only a high-purity oxygen gas and without including a nitrogen gas in a conventional ozone generator, but the result was that only a small amount of ozone was generated. The reason for this is presumably as follows. Oxygen molecules in the raw material gas have a continuous spectrum of light absorption with ultraviolet light at a wavelength of 245 nm or less (ultraviolet wavelength of 130 to 200 nm), and as a result of the oxygen molecules absorbing excimer light that is ultraviolet light at 245 nm or less, the oxygen molecules are dissociated into oxygen atoms, and ozone is generated by three-body collision of oxygen atoms produced by dissociation, oxygen molecules, and a third material. This fact is known with excimer lamps or the like that emit ultraviolet rays. However, excimer light that is ultraviolet light at 245 nm or less is not emitted by silent discharge at pressures as high as a pressure of 1 atmosphere or more caused primarily by an oxygen gas as in the ozone generator. Therefore, the reaction constant for the reaction process of dissociation of oxygen atoms and ozone generation by silent discharge light is very small, and it seems unlikely that a highly concentrated ozone gas of several % or higher will be generated by the reaction.
Accordingly, as a conventional method for supplying ozone to multiple ozone treatment apparatuses, for example, an ozone gas supply system as disclosed in Patent Literature 1 has been used in which a raw material gas that is a raw material oxygen gas containing a nitrogen gas of several thousand ppm or more, or a raw material gas obtained by forcibly adding a nitrogen gas in an amount of several thousand ppm or more to a raw material oxygen gas is supplied to an ozone generator so as to generate highly concentrated ozone, and in order to supply an ozone gas to a plurality of ozone treatment apparatuses, the capacity of one ozone generator is increased and a piping system for outputting an ozone gas is separated into a plurality of pipes so that an ozone gas having a predetermined concentration and flow rate is output to each of the multiple ozone treatment apparatuses in a stepwise manner.